This invention relates to catalytic combustors, especially those made of a plurality of pieces of metal foil arranged in a stack.
It has been known to make catalytic combustors by providing one or more strips of metal foil, stacking and/or folding the strips to form a monolith, and coating all or part of the monolith with catalyst. Examples of such combustors are given in U.S. Pat. Nos. 4,576,800, 5,202,303, and 6,060,173, the disclosures of which are incorporated herein by reference.
Catalytic combustors typically include flat strips alternating with corrugated strips. The corrugated strips tend to prevent the monolith from collapsing, and the corrugations help to define a cross-section having a large number of channels or cells. Increasing the density of such cells improves the efficiency of the combustor, by increasing the effective area for contact between the combustion gas and the catalyst disposed on the cell walls. Thus, it is usually an object of designers of catalytic combustors to maximize the density of the cells.
However, there is a major practical limitation on the cell density obtainable with the structures described above. The various strips of the monolith must be fastened together so that they can withstand the gas pressure in the combustor without falling out. The preferred means of fastening is welding. But it becomes very difficult to weld a structure in which the cells are very small.
Typically, a tweezer welder is used to reach inside the cells after assembly of the stack, and to weld together the ends of all the foil pieces. But this method is difficult to use when the cell density is of the order of 300 cpsi (cells per square inch), and where the cell height is typically less than about 0.05 inches. In such conditions, the tweezer welder cannot enter the cell.
The present invention provides methods whereby a tweezer welder can be advantageously used to fasten the pieces of foil in a catalytic combustor, even for relatively high cell densities. The invention also provides combustor structures which readily lend themselves to such welding.